How do I fade a whole scene?
You can do this using shader node too(Procedurally)
Follow this node group:

Value node gives the fade effect. Play with the values to get the desired effect.
Result:

P.S. Make sure you join both the objects
P.P.S If you wanna change the axis on which the fading occurs, change the value of last greater than node with Y or Z output from separate XYZ node
BLEND FILE: https://pasteall.org/blend/e1e9617c0bed4c18b372d759462ffb94